NEGCACHE: initialize UPN negative cache as well
    
    UPNs are handled separately in the negative cache. To properly filter
    user names even in the case of the fallback to a UPN lookup the negative
    cahe for UPNs has to be initialized with the names from the filter_user
    option as well.
    
    If the name from the option is a short name it will be added to the
    negative UPN cache for each domain with the respective domain name. If
    the name from the option is fully-qualified it will be added as is to
    the negative UPN cache for each domain.
    
    Related to https://pagure.io/SSSD/sssd/issue/3978
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>